1. Field of the Invention
This invention relates to a charging member and device for imparting electrical charge to an object, typically a photoconductor drum for use in an electrostatic latent image forming process, for example, copiers and printers.
2. Prior Art
The conventional electrophotography as applied to copiers and printers involves the steps of uniformly charging a photoconductor on the surface, projecting an image from an optical system onto the photoconductor to form a latent image in an exposed area where the electric charge is erased, applying toner to the photoconductor to form a toner image, and transferring the toner image to a record medium, typically paper.
The first step of electrically charging the photoconductor typically employs a corona discharge system. The corona discharge system, however, is undesirable from the standpoint of safety and maintenance of the machine since it requires application of as high voltage as 6 to 10 kV. It also suffers from an environmental problem due to the emission of harmful substances such as ozone during corona discharge. There is a need for an alternate charging system capable of charging at a lower applied voltage than the corona discharge and minimizing emission of ozone and other harmful substances.
One exemplary alternative charging system is a contact charging system wherein a charging member having voltage applied thereto is brought in contact with an object to be charged such as a photoconductor, thereby charging the object. Known charging members for use in the contact charging system include rollers which are based on rubber, urethane foam or the like and covered with a resin layer of polyurethane, nylon or the like.
These prior art charging members, however, have the following drawbacks. (1) When a resin layer is formed on an elastic layer of foam by dissolving a resin such as polyurethane and nylon in an organic solvent and applying the solution by dipping or spraying, the foam of the elastic layer can be swollen with the organic solvent to induce irregularities on the surface. The organic solvent which has penetrated into the foam will blow off the coating upon drying, forming crater-like holes in the coating surface. The charging member thus loses surface smoothness, failing to provide a uniform supply of electric charge from the charging member to the object. (2) Resins such as polyurethane and nylon tend to receive toner residues which are left on the photoconductor without being transferred. Then the toner residues adhere to the charging member, eventually leading to defective images.
The problems of surface smoothness and residual toner may be somewhat overcome by forming an elastic layer from an elastic material other than foam. In this case, however, the charging member has increased hardness, failing to provide an appropriate nip width between the member and the object to be charged to maintain a desirable physical contact therebetween.
Also, prior art charging members are not necessarily satisfactory in charging ability and durability. For improving charging ability and durability, it is necessary to achieve a uniform supply of electric charge from a charging member to an object to be charged. To this end, it is crucial to achieve a more uniform electrical state or resistance of the charging member and a more uniform physical contact between the charging member and the object.
Although the uniformity of resistance can be accomplished by improving the dispersion of conductive substance, such an improvement is difficult in practice. The reason is that the resistance region frequently chosen for charging members is not a good conductor region where conductive substance forms a definite conductive path, but a semiconductive region. Of course, it is contemplated to improve the dispersion of conductive substance and hence, the uniformity of resistance by adding a dispersant. The use of such additives as dispersants, however, is not recommended because they give rise to problems of migration and contamination to the object to be charged.
It is thus difficult to improve the charging performance and durability of a charging member while satisfying both the electrical uniformity thereof and the uniformity of physical contact thereof with an object to be charged.